A unit cell of a fuel cell system includes a membrane electrode assembly (MEA) that is sandwiched by and between separators. When the separators are in the form of metal separators, a resin frame may be interposed between the MEA and each of the separators. Such unit cells are stacked together to form a fuel cell stack. A cell monitor for obtaining information about the fuel cell, for example, for measuring voltage, may be mounted on each cell (or unit cell) or a suitable number of cells of the fuel cell stack. JP-A-2001-256991 discloses a fuel cell in which a recess (terminal mounting hole) is formed in an edge portion of a separator, and a terminal of a cell monitor is inserted into the recess.
However, the fuel cell disclosed in the above-identified publication, in which the cell monitor is mounted on each unit cell, suffers from complexity and inefficiency with which the cell monitors are mounted on and demounted from the respective cells. Also, the separator in which the terminal mounting hole is formed needs to have a thickness larger than the diameter of the hole, and it is thus difficult to form the terminal mounting hole in a metal separator having a small thickness.